This invention relates to an optical fiber system including a polarization-maintaining fiber, to a method of manufacturing the optical fiber system, and to an optical sensor for sensing a variable physical parameter by the use of the optical fiber system. It should be noted here and throughout the specification that a polarization-maintaining fiber can allow light to pass therethrough with the planes of polarization of the light kept therein without substantial interference. Such a polarization-maintaining fiber is called a birefringent fiber because it has birefringence induced by the internal stress or an isotropic structure.
A conventional optical fiber system of the type described is applied to an interferometer, namely, an optical sensor for measuring or sensing a variable physical parameter, such as temperature, pressure, magnetic field, or the like, by making use of a phase difference between two fundamental polarization modes of light which are orthogonal to each other. Such an interferometer is simple in structure because two polarization modes can be transmitted through a single polarization-maintaining fiber. More specifically, the polarization-maintaining fiber has a guide portion for guiding the light beams without any interference and a sensing portion for sensing a variable physical parameter produced from an object, as will later be described with reference to one figure of the accompanying drawing. Both of the polarization modes are individually subjected to phase shifts during passage through the sensing portion. It is possible to measure a variation of the physical parameter by monitoring the phase shifts.
The interferometer is, however, disadvantageous in that both of the polarization modes are also subjected to phase shifts at the guide portion of the polarization-maintaining fiber when disturbance or perturbation noises take place at the guide portion. Such noises become serious as the guide portion is rendered long. Accordingly, it is difficult with the interferometer to precisely measure the variation of the physical parameter.
In the meanwhile, it is preferable that such an optical fiber system is applicable to a wide variety of uses.